Deformation of the southern San Andreas Fault System induced by lateral variations in crustal rigidity

To improve our understanding of crustal rheology influences on fault loading processes, we have developed a 4-D earthquake cycle model of the San Andreas Fault System (SAFS) that incorporates spatial variations in lithosphere rheology. We have added 2-D lateral variations in crustal rigidity to our existing deformation code (maxwell) that is used for simulating time-dependent deformation and stress in complex transform fault systems. We construct a high-resolution (500 m) earthquake cycle model of the SAFS that implements spatial variations in crustal rigidity and comprises variable slip and locking depths along 42 major fault segments. Secular deep slip is prescribed from the base of the locked zone to the base of the elastic plate while episodic shallow slip is prescribed from the historical earthquake record and geologic recurrence intervals. To simulate spatial variations in rigidity, we use heat flow as a proxy for the thickness of the elastic plate and implement a provisional heat flow model for southern California [W. Thatcher, SCEC Community Thermal Model Working Group, personal communication]. We coarsely simulate plate thickness variations by varying the rigidity to be inversely proportional to heat flow. Fault slip rates are constrained by new SCEC Community Geodetic Model (CGM) data; 3169 horizontal GPS velocity measurements, combined with over 53,000 line-of-sight (LOS) Sentinel-1A InSAR velocity observations, are used in a weighted least-squares inversion. We compare these results to those reported in Tong et al. [2014]. As expected, we find that a decrease in regional crustal rigidity results in an increase in deformation. For example, high heat flow in the Salton Trough substantially reduces the effective elastic plate thickness, simulated by a reduced regional rigidity. This in turn, further amplifies fault-parallel crustal velocities in the region, especially near the Imperial fault, resulting in a regional velocity increase of at least 5 mm/yr. Moreover, the effect of a reduced crustal rigidity in the Salton Trough region has significant seismic hazard implications, as seismic moment accumulation rates are significantly reduced on local faults.